1. Field of the Invention
Embodiments of the invention relate to the field of ion implantation. More particularly, the present invention relates to in-situ energy contamination detection in an ion implanter.
2. Discussion of Related Art
Ion implantation is a process used to dope ions into a work piece. One type of ion implantation is used to implant impurity ions during the manufacture of semiconductor substrates to obtain desired electrical device characteristics. Typically, arsenic or phosphorus may be doped to form n-type regions in the substrate and boron, gallium or indium is doped to create p-type regions in the substrate.
An exemplary high current ion implanter tool 100 is generally shown in FIG. 1 and includes an ion source chamber 102, and a series of beam line components that direct the ion beam to a wafer or substrate. These components are housed in a vacuum environment and configured to provide ion dose levels with high or low energy implantation based on the desired implant profile. In particular, implanter 100 includes an ion source chamber 102 to generate ions of a desired species. The chamber has an associated heated filament powered by power supply 101 to ionize feed gas introduced into the chamber 102 to form charged ions and electrons (plasma). The heating element may be, for example, an indirectly heated cathode.
Different feed gases are supplied to the source chamber to generate ions having particular dopant characteristics. The ions are extracted from source chamber 102 via a standard three (3) extraction electrode configuration used to create a desired electric field to focus ion beam 95 extracted from source chamber 102. Beam 95 passes through a mass analyzer chamber 106 having a magnet which functions to pass only ions having the desired charge-to-mass ratio to a resolving aperture. In particular, the analyzer magnet includes a curved path where beam 95 is exposed to the applied magnetic field which causes ions having the undesired charge-to-mass ratio to be deflected away from the beam path. Deceleration stage 108 (also referred to as a deceleration lens) includes a plurality of electrodes (e.g. three) with a defined aperture and is configured to output the ion beam 95. A magnet analyzer 110 is positioned downstream of deceleration stage 108 and is configured to deflect the ion beam 95 into a ribbon beam having parallel trajectories.
A magnetic field may be used to adjust the deflection of the ions via a magnetic coil. The ribbon beam is targeted toward a work piece which is attached to a support or platen 114. An additional deceleration stage 112 may also be utilized which is disposed between collimator magnet chamber 110 and support 114. Deceleration stage 112 (also referred to as a deceleration lens) is positioned close to a target substrate on platen 114 and may include a plurality of electrodes (e.g. three) to implant the ions into the target substrate at a desired energy level. Because the ions lose energy when they collide with electrons and nuclei in the substrate, they come to rest at a desired depth within the substrate based on the acceleration energy. The ion beam may be distributed over the target substrate by beam scanning, by substrate movement using platen 114, or by a combination of beam scanning and substrate movement.
Deceleration of the ions by one or more stages 112 may be required when forming devices with shallower junction depths, but at high current levels. A deceleration stage 112 is positioned reasonably close to the target substrate to reduce the distance the beam must travel at low energy where the efficiency of transporting the beam is poor. However, ions directed at a substrate may lose their charge in a charge exchange reaction with residual gas along the beam line. These ions, commonly referred to as “neutrals”, are unaffected by one or more of the deceleration stages 112 and impact the target substrate at a higher energy level. This higher energy level causes the ions to implant deeper in the target substrate than desired and is Energy Contamination (EC). In other words, EC occurs when a fraction of the ion beam that is at a higher energy level for a given implant recipe reaches the target substrate. This is particularly problematic when forming, for example, a gate metal implant, where avoiding contamination of the oxide beneath this gate is important due to the fragility of the oxide layer.
Currently, attempts have been made to suppress and or deflect ions at higher energy levels than desired from reaching the target substrate to avoid EC through the use of high energy filters disposed downstream of the deceleration stage. However, a drawback associated with these filters is that a decelerated, low energy ion beam is very difficult to transport even over small distances because it is subject to large space charge blow-up. Thus, transporting the beam through an energy filter will not only attenuate the high energy neutrals, but will also attenuate the desired ions and prevent them from reaching the target substrate with a desired energy and at a desired concentration. Also, only a limited amount of current may be transported through such a filter, often with significant degradation of beam parallelism.
Other known techniques for limiting EC include the use of an electrostatic or magnetic bend disposed between the deceleration stage and the analyzer magnet, increased gas pumping to limit the neutralization of beam ions by residual gas, an aperture and liner design to prevent neutrals formed by collisions with the structures inside the implanter from reaching the workpiece, and limiting the voltage allowed when running deceleration to reduce the implanted depth of the contaminant ions.
In U.S. Pat. No. 7,250,617 entitled “Ion Beam Neutral Detection” assigned to the assignee of the present invention, a system and method for measuring the current of secondary electrons emitted due to the impact of energetic neutral particles is disclosed. This system measures a current of the ion beam at the collector plate wherein different portions of the current are measured depending on a bias voltage provided to the chamber. However, this requires the measuring of the various components of the beam current including the lower energy ions as well as the desired energy level ions and subtracting these measurements to determine the EC. This method of quantifying the high energy neutrals is thus dependent on the subtraction of two relatively large numbers and the error in the result is compounded by the arithmetic operation with a resulting loss in accuracy. Furthermore, this method depends on measuring the secondary electrons emitted from a surface when an ion or neutral atom impinged on that surface. However, secondary electron yields are very sensitive to surface cleanliness and can vary unpredictably. Accordingly, an improved EC detection system and method which provides a more direct measurement of the high energy ions associated with EC is desirable.